Donald Trump has decided he will meet with North Korea's Kim Jong Un "by May", South Korea's national security director has said.

Chung Eui-yong said Mr Trump said "he would meet Kim Jong Un by May to achieve permanent denuclearisation" of the Korean peninsula.

The security director said Mr Kim told the South Koreans he is "committed to denuclearisation" and pledged that "North Korea will refrain from any further nuclear or missile tests".

The meeting would be the first of its kind between a leader of North Korea and a sitting president of the United States.

In a tweet, Donald Trump said "great progress" was being made, although sanctions will remain in place until an agreement is reached.

He said: "Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze.

"Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached. Meeting being planned!"

The latest round of sanctions to be imposed on North Korea by the US was announced on Wednesday, when the State Department revealed punitive measures in response to its finding that Pyongyang was behind the nerve agent assassination of Mr Kim's half-brother.

South Korean President Moon Jae-in says a planned summit between US President Donald Trump and North Korean leader Kim Jong Un will be a "historical milestone" that will put the denuclearisation of the Korean Peninsula "really on track".

Mr Moon in a statement read out by his spokesman on Friday also complimented Mr Trump for accepting Mr Kim's invitation for a summit, saying the president's leadership will be praised "not only by the residents of South and North Korea but every peace-loving person around the world".

Mr Moon is also preparing for a summit with Mr Kim at a border village between the Koreas in April.